1. Field of the Invention
The present invention relates to a communications system for transmitting and receiving a voice signal and a tone signal, and more particularly, to a communications system, a communication method, a wireless communications system, a wireless microphone, and a wireless receiver for transmitting and receiving a voice signal and a tone signal.
2. Description of the Related Art
There have been proposed a wide variety of wireless communications systems each comprising a wireless microphone transmission apparatus for inputting a voice therethrough, converting the voice into a voice signal, modulating the voice signal with a carrier signal to generate a radio microphone signal, and transmitting the radio microphone signal carrying the voice signal, and a wireless receiving apparatus for receiving the radio microphone signal carrying the voice signal transmitted by the wireless microphone transmission apparatus, demodulating the radio microphone signal to generate the voice signal, converting the voice signal into the voice, and outputting the voice therethrough, thereby omitting a cable connecting between the wireless microphone transmission apparatus and the wireless receiving apparatus.
In such a wireless communications system, the wireless receiving apparatus is provided with a squelch circuit which is operative to cut off the operation of outputting the voice if the signal level of the received voice signal falls too low or the signal level of the noise signal increases too high, thereby avoiding a situation where high noise signals are outputted in the absence of wanted voice signals. The operation of the squelch circuit for cutting off the operation of outputting the voice if the level of the received voice signal falls too low or the level of the noise signal is too high will be hereinlater referred to as a “squelch operation”. The wireless communications system controls the squelch circuit to avoid outputting high noise signals when the wireless microphone transmission apparatus is turned off and not transmitting any voice signal.
The radio microphone signal transmitted by the wireless microphone transmission apparatus may include a noise signal, a carrier signal, and a tone signal. The squelch circuit includes various types such as a noise squelch circuit for performing the squelch operation on the basis of the noise signal contained in the received radio microphone signal, a carrier squelch circuit for performing the squelch operation on the basis of the carrier component contained in the received radio microphone signal, and a tone squelch circuit for performing the squelch operation on the basis of the tone signal contained in the received radio microphone signal.
The noise squelch circuit and the carrier squelch circuit encounters drawbacks being inclined to operate malfunctions due to the fact that they are vulnerable to misidentifying noises unnecessarily emitted from electrical appliances such as a personal computer, a pachinko machine and an air cleaning device. The wireless receiving apparatus is therefore equipped with the tone squelch circuit in addition to the noise squelch circuit or the carrier squelch circuit to prevent from performing malfunctions caused by the noises emitted from the electrical appliances.
The wireless microphone transmission apparatus is designed neither to transmit any tone signal until a predetermined time elapses after the turn-on operation of the wireless microphone transmission apparatus, nor to turn off the power until a predetermined time elapses after the turn-off operation of the power switch, in order to cut off noises occurred when the wireless microphone transmission apparatus is turned on and off.
Referring to FIGS. 21, 22, and 23 of the drawings, there is shown a conventional wireless communications system 700 of this type comprising a conventional wireless microphone transmission apparatus 710 and a conventional wireless receiving apparatus 740. In this conventional wireless communications system 700, the conventional wireless microphone transmission apparatus 710 is adapted to transmit a tone signal when the wireless microphone transmission apparatus 710 is turned on. The conventional wireless receiving apparatus 740 is equipped with a squelch circuit section 727 operative not to output a voice signal when no tone signal is received from the conventional wireless microphone transmission apparatus 710.
The conventional wireless microphone transmission apparatus 710 is shown in FIG. 21 as comprising a voice inputting section 701 for inputting a voice information therethrough and converting the voice information into a voice signal, a tone signal generating section 702 for generating a tone signal, a mixing section 703 for mixing the voice signal and the tone signal to generate a transmission signal, an oscillation section 704 for generating a carrier signal, a modulating section 705 for modulating the transmission signal with the carrier signal, a high-frequency amplifying section 706 for amplifying the modulated transmission signal to generate a radio microphone signal on a particular microphone frequency channel, and a transmission antenna 707 for transmitting the radio microphone signal on the particular microphone frequency channel to the conventional wireless receiving apparatus 740.
The conventional wireless receiving apparatus 740 is shown in FIG. 22 as comprising a receiving antenna 721 for receiving radio microphone signals, a high-frequency amplifying section 722 for amplifying the received radio microphone signals, a frequency conversion section 723 for selecting a radio microphone signal on the particular microphone frequency channel from among the radio microphone signals and converting the selected radio microphone signal on the particular microphone frequency channel into a intermediate-frequency microphone signal, an intermediate-frequency amplifying section 724 for amplifying the intermediate-frequency microphone signal, a decoding section 725 for generating a microphone signal from the intermediate-frequency microphone signal thus amplified, a band pass filter (referred to as “BPF”) 726 for filtering the voice signal therethrough from the microphone signal inputted by the decoding section 725, a squelch circuit section 727 for cutting off or passing through the voice signal inputted from band pass filter 726, a low-frequency amplifying section 728 for amplifying the voice signal, a crystal filter 729 for filtering the tone signal therethrough from the microphone signal inputted by the decoding section 725, and a squelch circuit control section 730 for controlling the squelch circuit section 727 to have the voice signal cut off or pass through on the basis of the tone signal inputted from the crystal filter 729. The conventional wireless receiving apparatus 740 thus constructed can extract the voice signal and tone signal from the received radio microphone signal to output the voice signal and tone signal separately.
In the conventional wireless receiving apparatus 740 thus constructed, the squelch circuit control section 730 is operative to control the squelch circuit section 727 so as to output the voice signal only when the tone signal is filtered through by the crystal filter 729. Otherwise, the squelch circuit control section 730 is operative to cut off the electrical connection between the BPF 726 and the low-frequency amplifying section 728 to prevent any noise signal from being outputted.
FIG. 23 is a graph showing an example of a transmission signal, i.e., a radio microphone signal including a voice signal and a tone signal transmitted and received in the conventional wireless communications system 700. As will be understood from FIG. 23, the frequency of the tone signal is generally chosen outside of the frequency range of the voice signal.
The wireless communications system is recently utilized in an occasion of making a presentation. During the presentation, a presenter is required to control operations of various devices such as, for example, the sound level of the wireless receiving apparatus which receives and amplifies the presenter's voice, the operation of the recording apparatus which records the presenter's presentation, and replaying and adjusting operations of the video tape recorder. Thus, the presenter tends to utilize the wireless communications system not only for amplifying the presenter's voice but also for controlling the operations of the other devices. Some types of the wireless communications system use the tone signal for conveying control data in order to control the operations of various devices.
Such types of wireless communications system include two types consisting of a first type of wireless communications system for amplitude-modulating a tone signal to convey a control data, and a second type wireless communications system for generating a tone signal having a plurality of tones each having a signal level and repeated at predetermined time intervals to convey a control data. The wireless communications system of the second type is disclosed, for example, in Japanese Patent Laid-open Publication No. 6-113376 as comprising a wireless microphone transmission apparatus including a tone generating section for generating a tone signal having a plurality of tones repeated at predetermined time intervals and a wireless receiving apparatus. “A tone signal” is intended to mean “a plurality of tones having a signal level” oscillated or repeated at “time intervals”. The wireless microphone transmission apparatus disclosed therein is adapted to transmit a radio microphone signal carrying a tone signal indicative of a control data having a plurality of tones repeated at predetermined time intervals, and the wireless receiving apparatus disclosed therein is adapted to receive the radio microphone signal to extract the tone signal and detect the control data, thereby making it possible for the wireless communications system to have a tone signal transmitted and received.
The above wireless communications system, however encounters a drawback that the squelch circuit may cut off the wanted voice when no tone signal happens to be detected for a certain time interval as a result of an incidental combination of tone signals.
The above wireless communications system encounters another drawback that the wireless receiving apparatus of the above system is required to include one extraction circuit for extracting one type of tone signal having tones repeated at predetermined time intervals. This means that the wireless receiving apparatus of the above system is required to include two extraction circuits for extracting two types of tone signals each having tones repeated at respective predetermined time interval. This leads to the fact that the wireless receiving apparatus of the above system is required to include the same number of extraction circuits as the number of types of tone signals each having tones repeated at respective predetermined time interval.
Furthermore, the above wireless communications system encounters another drawback that the sensitivity is deteriorated and the transmission distance is shortened because the signal to noise ratio, i.e., SN ratio is worsened whenever the tone signals are amplitude-modulated.